A multimodal imaging system may comprise PET, SPECT, and CT combined in the same imaging system. Image data acquired from a subject, for example a patient or an animal, may in such as multimodal system derive from all three techniques or a combination thereof.
In CT, an external x-ray source is caused to be passed around a subject, for example an animal or a patient. Detectors around the subject then respond to x-ray transmission through the subject to produce an image of an area of study. Unlike PET, which is an emission tomography technique because it relies on detecting radiation emitted from the subject, CT is a transmission tomography technique which utilizes a radiation source external to the subject.
In SPECT, gamma rays are detected by at least one gamma camera rotating around the subject. Projections are acquired at defined points during the rotation and this information may be presented as cross-sectional slices through the subject, but can be freely reformatted or manipulated as required. The acquired multiple 2-D images, also called projections, may be processed by a computer applying a tomographic reconstruction algorithm to the multiple projections, yielding a 3-D dataset. This dataset may then be manipulated to show thin slices along any chosen axis of the body. SPECT acquisition is very similar to planar gamma camera imaging and the same radiopharmaceuticals may therefore be used. This also allows for possibilities to combine SPECT with other medical imaging processes.
PET is a branch of nuclear medicine in which a positron-emitting radiopharmaceutical is introduced into the body of the subject. Positrons emitted interact with free electrons in the area of interest, resulting in annihilation of the positron. This annihilation yields the simultaneous emission of a pair of photons approximately 180 degrees apart. The radiation resulting from this annihilation is detected by a PET scanner. More specifically, each of a plurality of positrons reacts with an electron in what is known as a positron annihilation event, thereby generating a coincident pair of 511 keV gamma rays which travel in opposite directions along a line of response (LOR). After acquiring these annihilation “event pairs” for a period of time, the isotope distribution in a cross section of the subject can be reconstructed.
A medical imaging device, such as a PET tomograph, is used to detect the positron annihilation events and generate an image of at least portions of the subject from a plurality of detected events. The PET tomograph may comprise a plurality of radiation-sensitive PET detectors arrayed about an examination region through which the subject is conveyed. The PET detectors typically comprise crystals and photomultiplier tubes (PMTs) or Avalanche Photo Diodes (APDs). The detector crystals, referred to as scintillators, convert the energy of a gamma ray into a flash of light that is sensed by the detector, PMT or APD. In coincidence mode a gamma ray pair detected within a coincidence time by a pair of PET detectors is recorded by the PET scanner as an annihilation event. Due to the approximate 180 degree angle of departure from the annihilation site, the location of the two detectors registering the event define the LOR passing through the location of the annihilation. Detection of the LORs is performed by a coincidence detection scheme. A valid event line is registered if both photons of an annihilation are detected within a coincidence window of time. Coincidence detection methods ensure that an event line is histogrammed only if both photons originate from the same positron annihilation. The observed events are typically sorted and organized with respect to each of a plurality of projection rays. By histogramming these LOR, a “sinogram” is produced that may be used by, for example, a process to produce a three dimensional image of the activity. All events occurring along each projection ray may be organized into one bin of a three-dimensional sinogram array. The array may be stored in a computer-readable memory media. The sinogram data is then processed to reconstruct an image of the scanned volume.
Detection of a gamma in one of the crystals may start the events processing chain in several blocks at the same time depending on the sharing design used. A block may contain four PMTs or APDs. The PMTs or APDs convert the light signal from the scintillator into an electrical signal and the connected read-out electronics processes the event by using, for example, printed circuit boards. Accepted events are then transferred to a coincidence controller for further processing. The printed circuit board may have as a main purpose to condition, digitise and process incoming analog pulses from PMT or APD based PET or SPECT detectors.
These printed circuit boards may be analog. Consequently, the reprogramming and adaptation of a medical imaging device, especially the printed circuit boards processing the analog pulses from PMT or APD based PET or SPECT detectors, is cumbersome, if not impossible. It would be desirable to have a medical imaging device that could easily implement and/or test new algorithms or methods for processing the pulses without having to change the hardware.
Another problem is that the high count rate of the gamma causes a problem called pulse pileup, in which two events arrive at the detector so close in time that they produce signals that cannot be separated thoroughly by the electronics. The likelihood of pileup of events increases in a medical imaging device with large block detectors. Developments within the industry lean towards PETs and SPECTs having large block detectors. Electronics able to cope with the increase in events rate is sought after.
A further problem underlying event processing is to obtain a good energy and/or positioning performance from detectors while maintaining high stability across count rates. Additionally, it is desirable to avoid the cumbersome arrangements from a technical and/or economical point of view. Further, low energy consumption and production costs of a medical imaging device are desirable.